The present disclosure concerns a method and a device for digital printing to a recording medium with liquid ink.
In digital printing, both liquid toner and liquid colors are used as ink.
Devices for digital printing to a recording medium with liquid toner are liquid toner printing apparatuses, in which toner particles are applied to the recording medium to be printed to with the aid of a liquid toner. Such devices are known from DE 10 2010 015 985 A1, DE 10 2008 048 256 A1, DE 10 2009 060 334 A1 or DE 10 2012 111 791 A1. For this, a latent charge image of a charge image carrier is inked by means of electrophoresis with the aid of a liquid toner. The toner image created in such a manner is transferred to the recording medium indirectly (via a transfer element) or directly. The liquid toner has toner particles and carrier fluid in a desired ratio. The toner particles are suspended in the carrier fluid. This allows the use of small toner particles with a diameter of less than 8 μm, for example. If such small particles are handled as powder, they pose a health hazard. In contrast to this, if they are suspended in a carrier fluid, no health hazard exists. The use of such small toner particles on the one hand allows a print image with very high resolution since the toner particles are smaller than given conventional electrophoretic printing methods in which no carrier fluid is used. Furthermore, the layer thickness of the toner particles on the recording medium is less. This is advantageous in particular when multiple colors are printed atop one another. The toner particles incur the greatest costs in the printing process. The smaller the printed toner quantity, the lower the costs as well.
Mineral oil can be used as carrier fluid. In order to provide the toner particles with an electrostatic charge, charge control substances are added to the liquid developer. Further additives may additionally be added, for example in order to achieve the desired viscosity or a desired drying behavior of the liquid developer.
The toner particles are comprised of wax and color particles. In the fixing process, the recording medium with the applied toner particles is heated, whereby the carrier fluid is vaporized. The toner particles are hereby also heated and thermoplastically deformed. The particles flow into one another and bind to the recording medium. The heating of the recording medium thus simultaneously serves to fix the toner particles onto the recording medium and to dry the recording medium.
The carrier fluid vaporized in the fixing station mixes with air and thus forms a flammable gas, is designated in the following as “combustible gas”. This combustible gas is supplied to a combustion chamber and burned there. Via the burning of the combustible gas, a waste gas is generated that is not flammable and in which toxic components of the combustible gas are converted into non-toxic components. The combustible gas is heated and the waste gas is cooled with a heat exchanger. If a heat exchanger with high efficiency is used, a bypass line for the waste gas is then provided with which a portion of the waste gas is supplied past the heat exchanger to a chimney. Given fluctuations in the input of the carrier fluid into the combustible gas, in the short term a large quantity of heat may hereby be removed from the system as a whole since the regulation of the air supply for the combustible gas is too slow in order to be adapted to a rapidly changing vapor quantity of carrier fluid.
If a heat exchanger with low efficiency is used, it is then in fact possible to direct a large quantity of hot waste gas through the heat exchanger. In order to be able to compensate for fluctuations in the input of the carrier fluid, given such a device the burner is in principle operated with a high proportion of fuel that may be reduced given a short-term increase in carrier fluid vapor or increased again given a decrease in carrier fluid vapor. The vapor quantity is thus compensated by varying the supply of fuel. In this embodiment, the fuel consumption is significantly greater than given the embodiment with the bypass line as explained above.
The bypass line may be opened and closed quickly with corresponding valves. Since the uncooled waste gas is supplied to the bypass line, it must be designed for correspondingly high temperatures. This also applies to the valves. This is technically complicated and generates correspondingly high costs.
In offset printing, solvents are vaporized in a fixing station, which solvents—with supplied air—form a combustible gas and are similarly thermally heated in a combustion chamber. However, in offset printing a specific print image is often printed successively, such that the entry of solvent is essentially constant. In offset printing, there is hereby not the problem of varying composition of the combustible gas.
In offset printing, only the solvent is dried off from applied color particles, but these are not thermoplastically deformed. A print image generated in digital printing with the liquid toner explained above is significantly more stable than a print image generated in offset printing since the digital print image generated by means of liquid toner can no longer be dissolved due to the thermoplastic deformation.
It is also known that ink may have a flammable solvent or also a mineral oil that must be prepared in the printing process, similar to the carrier fluid explained above.
The exemplary embodiments of the present disclosure will be described with reference to the accompanying drawings.